Cultures of rat pheochromocytoma (PC12) cells treated with beta-nerve growth factor (NGF) for up to 15 days continue to synthesize DNA. The present study compares the extent of maintained DNA synthesis in cells with and without processes and asks whether the observed DNA synthesis in differentiated PC12 cells reflects either the continued division of the cells or the formation of polyploid cells, or both. PC12 cells were grown on tissue coverslips for various lengths of time with or without 50 ng/ml of beta-NGF and then assayed for DNA synthesis by [3H]thymidine labeling and autoradiography. In 8-day-old control cultures (no NGF), 30% of the cells had labeled nuclei after a 2-hr [3H]thymidine pulse. In contrast, in cultures treated for 8 days with NGF, only 7% of the cells were labeled (i.e., still synthesizing DNA). The fractions of process-bearing and non-process-bearing cells with labeled nuclei were identical. Even after 14 days in NGF, 7% of the cells with neurites were still synthesizing DNA during any 2-hr period. With continuous [3H]thymidine labeling in the presence of NGF from 8 to 13 days, nearly 70% of the cells with neurites were labeled. The presence of neurites induced by NGF does not preclude continued (albeit reduced) DNA synthesis in these PC12 cells. To determine the fate of this newly synthesized DNA, nuclei extracted from NGF-treated PC12 cells were analyzed for the cellular distribution of DNA by combined propidium iodine staining and flow microfluorimetry. NGF treatment resulted in a 3-fold increase in the number of G2+M/4N cells along with the appearance of 8N cells.(ABSTRACT TRUNCATED AT 250 WORDS)